In powder metallurgy (PM), there are several ways of introducing alloying elements into a PM material in order to adjust a certain alloying element content. Each alloying route has its advantages and disadvantages. Master alloys (MA), powders with a high content of typically several alloying elements, can be added in small amounts to a base powder, especially to introduce oxygen sensitive elements such as Cr, Mn, and Si. In addition, the master alloy can be designed in such a way that a liquid phase is formed intermediately during the sintering process to improve the distribution of alloying elements in the material and to accelerate homogenization. In this study, such master alloys were combined with pre-alloyed base powders to form hybrid alloyed mixtures with the aim of improving the material‘s sinter hardenability. The hybrid alloys were compared with mixtures of master alloy and plain Fe as reference material. The sinter hardenability of all materials was determined by generating CCT diagrams recorded with 13 different cooling rates. These were verified by metallographic cross-sections of specimens treated at common cooling rates of 3 and 1.5 K/s and subsequent hardness measurements of the microhardness (HV 0.1) of the microstructural constituents and the apparent hardness (HV 30). ◼

The application of “masteralloys” as alloying carriers in Powder Metallurgy (PM) steels enables the introduction of highly oxygen‑sensitive alloying elements for advanced PM‑steels, by using a tailored liquid phase. The characteristics of the liquid and its interaction with the iron base powder are determining factors for final microstructure and dimensional behaviour. In this study, theoretical calculations and experimental findings are presented for the masteralloy systems Fe_Mn_Si_C, Fe_Cr_Si_C and Fe_Mn_Cr_Si_C. Lowered melting temperatures and narrow melting temperature intervals could be achieved. The interaction between Fe base material and the masteralloys was studied by infiltration and DTA experiments. It was found that by adjusting the C and Si content in the masteralloy, liquids with widely varying properties could be obtained. This might be a key for tailoring microstructures, properties and dimensional stability of advanced PM‑steels.

Sinter hardening is a powder metallurgy processing route that combines the sintering and the heat treating processes in one step by gas quenching the components immediately after they have left the high temperature zone of the furnace. It is both economically attractive and ecologically beneficial since it renders deoiling processes unnecessary. The slower cooling rates associated with gas compared to oil quenching however requires special alloy concepts different to those known from wrought steels. In the present study it is shown that by admixing atomized masteralloy powders consisting of suitable combinations of Mn, Cr, Si, Fe and C to base iron or pre-alloyed steel powders, sinter hardening PM steel grades can be produced that transform to martensitic microstructure at cooling rates of 2-3 K/s as typical for industrial sinter hardening. This is confirmed by CCT diagrams and hardness measurements. However, metallographic investigations are also necessary because in sintered steels, the cores of the largest base powder particles are alloyed very slowly during sintering and therefore tend to result in soft spots in the sinter hardened microstructure which are mostly not discernible in the CCT diagrams. Here, even slight pre-alloying of the base powder with Mo and/or Cr is helpful, both increasing the hardenability of the steels compared to base plain iron and avoiding soft spots in the microstructure.

Powder metallurgy (PM) offers several variants to introduce alloying elements for establishing the desired final composition. One route is the master alloy (MA) approach. The composition and the elements contained in the MA can be adjusted to obtain a liquid phase that penetrates through the interconnected pore network and thus enhances the distribution of the alloying elements and the homogenization of the microstructure. Such a liquid phase is often of a transient character, and therefore the amount of liquid formed and the time the liquid is present during the sintering are highly dependent on the heating rates. The heating rate has also an impact on the reaction temperatures, and therefore, by properly adjusting the heating rate, it is possible to sinter PM-steels alloyed with Fe-Cr-Si-C-MA at temperatures below 1250 °C. The present study shows the dependence of the melting regimes on the heating rate (5, 10, 20, 120 K/min) represented by “Kissinger plots”. For this purpose, liquid phase formation and distribution were monitored in quenching dilatometer experiments with defined heating up to different temperatures (1120 °C, 1180 °C, 1250 °C, 1300 °C) and subsequent quenching. Optimum sintering conditions for the materials were identified, and the concept was corroborated by C and O analysis, CCT diagrams, metallographic sections, and hardness measurements.

Among the various alloying techniques used in powder metallurgy, the masteralloy concept has been known for a long time. However, its use for production of ferrous precision parts has been hampered by several obstacles such as poor output of the useful fine fractions, high tool wear and slow homogenization kinetics of the alloy elements in the matrix. On the other hand, the masteralloy concept is particularly interesting for introducing cost-effective alloy elements such as Cr, Mn and Si since the masteralloy approach at least alleviates the problems caused by the high oxygen affinity of these elements. In the present study it is shown that recent developments have given a boost to this classical concept, one of these developments being powder manufacturing by high pressure water atomization which dramatically increases the yield of fine masteralloy fractions. The other progress is availability of thermodynamic software that enables defining masteralloy compositions with low melting range and thus fast homogenization also at moderate sintering temperatures. Combined, these new developments open the door for implementation of the masteralloy route in large scale PM parts production.